Hitherto, with respect to the dimensional measurement of an article obtained by the extrusion molding, an on-line measurement could be carried out only in a manual measurement of the width thereof by an operator using a measuring tape, and the profile thereof was measured in general by an off-line measurement wherein a cut section obtained by cutting an extruded article was transcribed on paper for subsequent measurement of the cross sectional shape.
However, in the above manual method of on-line measurement, there were problems that other dimensions than the width could not be measured, and that the obtained value was not reliable because of a manual operation. As for the above off-line measurement of the profile by cutting the subject article into a section, there were problems that it is difficult to avoid errors caused by deformation, for example, shrinkage due to temperature change, and that the measurement was usually associated with an individual difference and measuring error because of a manual measurement using a dial gauge or the like.
In view of the above situation, recently various new methods for shape measurement are under development. For example attempts have been made for establishing shape measuring methods including a first method wherein a prespecified designed shape of the cross section of an article is illustrated on a screen of a display by utilizing, for example, a computer system and the actual shape of the cross section obtained by a laser beam scanning at a predetermined portion of the article is also illustrated on the same screen of the display in the same scale and the same view, then the displacement between the two figures illustrated on the display, is measured. A second profile measuring method is where the designed shape showing the contour line of an article is illustrated on a screen of a display based on the design shape data. The actual contour line obtained by image pick-up of the subject article using ordinary light in the same scale and the same view as the designed shape is also illustrated on the same screen of the display. Then the displacement between the two contour lines is measured.
Thus many efforts have been made to solve the problems of the prior art, but in spite of the efforts, an advent of a simple, reliable system for shape measurement which, unlike the above conventional method, does not use a laser beam or the like has been eagerly awaited.